1) Field of the Invention
The present invention relates to technology for enhancement of the weld strength of hermetic cells.
2) Description of the Related Art
Rectangular hermetic cells are easy to mount inside electronic appliances and thus used for power sources therefor.
These hermetic cells are prepared by engaging a top plate assembly with an opening portion of the can and laser welding the portion of engagement in a hermetic manner. If the weld strength is not sufficient, cracking may occur on the melt mark when the cell is dropped, which causes leakage of the electrolytic solution and infiltration of oxygen or moisture into the cell, thus adversely affecting degraded cell performance.
A possible solution to this problem is to increase the laser strength to enlarge the depth of melting of the melt mark caused by the laser, thereby enhancing the weld strength by enlarging. However, this poses the problems of occurrence of sputtering and early degradation of the laser apparatus.
The technology for enhancement of the weld strength is disclosed in Japanese Patent Application Publication No. 2002-292486 (patent document 1), Japanese Patent Application Publication No. 2006-19089 (patent document 2), and Japanese Patent Application Publication No. 2006-260883 (patent document 3).
Patent document 1 discloses a technique to make the reflectivity of laser light lower on the sealing cover than on the can. This inhibits degradation of the sealing performance of the laser welded portion.
Patent document 2 discloses a technique concerned with the spot melt mark made as a result of laser welding of the can and the sealing plate. A cross section of the melt mark parallel to the length of the can and providing the shortest distance between the outer surface of the can and the sealing plate is protruded in a mountainous shape in the outward direction relative to the can from the bottom of the spot melt mark, and the peak of the mountainous protrusion is positioned further toward the inner surface of the sealing plate than a plane encompassing the outer surface of the sealing plate. This enhances the weld strength.
Patent document 3 discloses a technique including the steps of: preparing a rectangular case having an opening portion with a cross section such that the outer surfaces of the side walls of the case defining an opening portion are linear, and that in the side walls having the longest linear outer surfaces among the side walls defining the opening portion, the center portion of each side wall is thicker than both ends of the side wall; locating a sealing member to a position to seal the opening portion, the sealing member being large enough to cover the opening portion; and welding the sealing member onto the case by irradiating the boundary between the sealing member and the side walls at the periphery of the opening portion with high-density energy from a side direction of the case while varying the depth of melting in the thickness direction of the side walls in accordance with the thickness of the side walls. This enhances the weld strength.